1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
2. Description of the Related Art
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc., with a view to improving the ride quality and the steering stability.
In general, this type of hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston which has a piston rod connected thereto and which is slidably fitted in the cylinder to define therein two cylinder chambers. The two cylinder chambers are communicated with each other by a main hydraulic fluid passage and a bypass passage. The main hydraulic fluid passage is provided with a damping force generating mechanism (including an orifice, a disk valve, etc.) for generating relatively large damping force. The bypass passage is provided with a damping force generating mechanism for generating relatively small damping force, and with a damping force control valve for opening and closing the bypass passage.
With the above-described arrangement, when the damping force control valve is opened, the hydraulic fluid in the cylinder is induced to flow mainly through the bypass passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating relatively small damping force. Accordingly, the damping force characteristics are "soft" during both the extension and contraction strokes. When the damping force control valve is closed, the hydraulic fluid in the cylinder is induced to flow through only the main hydraulic fluid passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating relatively large damping force. Accordingly, the damping force characteristics are "hard" during both the extension and contraction strokes. Thus, the damping force characteristics can be changed by opening and closing the damping force control valve.
The above-described damping force control valve has a guide member and a shutter adapted to be slidable relative to it. A guide port provided in the guide member and a shutter port provided in the shutter are arranged to form a variable orifice. When the hydraulic fluid passage area defined by the orifice is small, the hydraulic fluid passing through it flows out in the form of a jet. Since the jet of hydraulic fluid is spouted at an angle to a direction perpendicular to the direction of sliding of the shutter, hydrodynamic force acts on the shutter, producing an adverse effect on the control of the sliding movement of the shutter.